I know it would never happen, but I'd trade Rielly before any of Muzzin, Sandin, Lehtonen, or Dermott.
In terms of value, I think you're losing on any trade involving Dermott. Sandin has huge value, but if he's as good as everyone thinks he could be, you wouldn't want to deal him anyways. Muzzin is our best D, he's not going anywhere.
I would do a Rielly, Fred, + for Gibson, Drysdale any day, but I doubt the Ducks would.
Not sure what Dahlin is worth these days, and if Buffalo is in win now mode, but I'd offer Rielly++
Would also offer him up to Dallas in return for Harley plus some combo of Bourque/Dellandrea.
Basically I'd rather have a 2-Way D instead of Rielly. After watching Muzzin, Holl, Brodie play a really good 200 foot game, it's obvious that those are the ones you want in the ice during all situations. Add to the fact Muzzin if producing better than Rielly 5 on 5 despite way harder competition, it's a not brainer that if you can get a comparable D to Rielly, but will give up a bit of offence, for defence you have to do it. Plus right now Rielly is massively overrated, so let some other GM get stuck holding the bag.